Image forming apparatus

ABSTRACT

An image forming apparatus includes an image forming station for forming a pattern image on a recording material; an image reading station for reading a pattern image formed on a recording material by the image forming station; and a tone gradation corrector for calculating a halftone dot area property on the basis of the density of the pattern image read by the image reading station and for executing tone gradation correction in the image forming station on the basis of the calculated halftone dot area property.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of International Application PCT/JP2009/065459having an international filing date of 28 Aug. 2009, which is based onand claims priority from Japanese Patent Application No. 2008-220568filed on Aug. 28, 2008. The contents of each, in their entirety,including the drawings, claims, and the specification thereof, areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an image forming apparatus for formingan image on a recording material, such as a copying machine or a printerof an electrophotographic type.

BACKGROUND ART

As shown in FIG. 30, output information (RGB image gray gradation image,CMYK image data) from a personal computer (PC) is supplied to the imageprocessor provided in or connected with the image forming apparatus. Inthe image forming station, the output information from the PC isprocessed by a gamma correction portion for making correction inaccordance with the gamma condition set by the printer driver dependingon the user preferable, a converting portion to L*a*b* (only L* forgray), a converter to), C′M′Y′K′.

A color space conversion from a many dimension to a many dimension, forexample, from RGB to L*a*b* or the like, is effected by the imageprocessor using a multi-dimensional conversion table information of anICC profile.

Referring to FIG. 31, there is shown a color setting portion of aprinter driver which is a user interface (UI) provided for the user seta desired tone gradation. On the basis of the set point information, thegamma correction portion effects a gamma conversion using aunidimensional LUT.

Referring to FIG. 30, the image data converted to the C′M′Y′K′information is subjected further to a unidimensional LUT process by theengine tone gradation corrector. The engine tone gradation corrector iseffective to keep the constant tone gradient of the printer engine. Thestate of the engine is detected using a patch image outputted on therecording material, that is, the sheet, and a unidimensional LUT isproduced so as to provide a predetermined tone gradation curve. Astabilization control in which a patch outputted on the sheet thereafteris called automatic tone gradation correction.

A pseudo-half-tone processing portion functions to reproduce a half-toneusing a dither method or the like. Since the engine tone gradationproperty is different depending on the kind (line number and/or dotgrowth method) of the half-tone, the tone gradation corrector isprovided with a LUT for each of half-tone processes.

A technique assuring the stabilization property of color using theengine tone gradation corrector has been proposed.

Japanese Laid-open Patent Application Hei 1-309082, for example, a patchpattern for toner content detection formed on the photosensitive memberis read by a density sensor, and the read information is fed-back to thetoner content controller in the developing device to provide a properdensity.

Generally, such a toner patch is easy to produce and erase, but what isobtained thereby is only density information prior to image fixing, andtherefore, when the control based on the toner patch is carried out, theinfluence of the secondary transfer or the fixing step are notreflected.

Japanese Laid-open Patent Application Hei 1-309082 proposes that animage is read by a reader portion of a copying machine main assembly,and on the basis of the result of the reading, the image formation iscontrolled. With this method, the output print provided by fixing thetoner on the sheet is read by the reader portion.

As a result of printing on standard, non-standard paper A (smooth thinpaper), non-standard paper B (rough thick paper) under constant imageforming conditions (charging, latent image and developing conditions),the density values detected by the reader portion are as shown in FIG.32. On the basis of this information, a LUT shown in FIG. 33 is producedto provide a density.

The read density value is different depending on the paper surface ofproperty, whiteness degree, thickness or the like, even if the toneramount thereon is the same, and therefore, the LUT varies. The LUTprepared on the basis of the non-standard paper B is lower in themaximum density than the target, and therefore, after input value of240, data of 255 is used, for all. Therefore, over 240th level, thesignal value exhibits no tone gradation. This is critical defect for theuser who takes the tone gradient in the shadow portion in such a rangeimportant.

In addition, when the LUT is prepared on the basis of the non-standardpaper A, the signal of 255 is outputted around the level of 240.Therefore, in the case of a signal indicative of solid image (halftonedot area percentage is 100%), a half-tone process pattern appears withthe result that quality deterioration, and/or jaggy patterns (screenpatterns) appear at edge portions. Thus, the quality of letters and/orlines deteriorates.

In order to solve such a problem, Japanese Laid-open Patent Application2006-165752 proposes that when the input signal is 255 (halftone dotarea percentage is 100), the OUT side of the LUT is made 255, thusproviding a certain degree of tone gradation.

FIG. 34 shows a LUT prepared on the basis of the non-standard paper B,and a LUT prepared by the method disclosed in Japanese Laid-open PatentApplication 2006-165752. It will be understood that the LUT exhibits asmooth tone gradation. FIG. 35 shows a reproducibility of the densityusing the LUT. In the shadow portion, the density is different from thetarget density, but the tone gradient is slightly improved.

However, with such technique, the tone gradation change of the shadowportion is so intense that an inflection point appears at (a) in FIG.35, and therefore, smooth tone gradation property is not provided.Because of this, some professional users are not satisfied.

As another technique relating to the automatic tone gradationcorrection, Japanese Laid-open Patent Application 2004-289200 andJapanese Laid-open Patent Application 2006-222804 discloses that a patchimage is printed on the non-standard paper using tone gradationcorrection data prepared on the basis of the standard paper, and theresult of reading the image is used for a correction target. Theautomatic tone gradation correcting method is proposed as being free ofthe difference in the density or surface property of the paper per se.

In many cases, the density in the foregoing automatic tone gradationcorrections is not an absolute density but is a paper based density(Null Density, relative density). In the following, the paper baseddensity is called relative density. Relative density=absolute density ofimage−absolute density.

The density management method in the relative density is stipulated inISO13656 or the like, and is an ordinary reference in the printing fieldUsing the relative density, it is unnecessary to change the targetdensity even when the printing is carried out on various kinds of paper,and therefore, the relative density has been used for management of theprinting machines in printing companies

Also in the automatic tone gradation correction for an image formingapparatus, the automatic tone gradation correction has been carried outon the basis of the paper based density in view of variations of thereading apparatus and change with time thereof, differences of the paperdensity due to the differences of the paper lot difference, or the like

However, in the case of using the automatic tone gradation correctiondisclosed in Japanese Laid-open Patent Application 2004-289200 andJapanese Laid-open Patent Application 2006-222804, the user has toprepare standard paper beforehand. In addition, the user has to be awareof the type of the non-standard paper and has to always carry out thetone gradation correction for the non-standard paper this means thatwhat results is mere replacement of the standard paper with thenon-standard paper. There is a liability that the non-standard paperregistered on the basis of the standard paper may become out of stock.In such a case, the problem arises, because the user encounters thedifference in the surface property, the whiteness degree, thickness,and/or grain of the paper due to lot difference, in addition, thedifference in the whiteness degree, the paper per se due to theatmospheric corrosion resistance.

When a plurality of kinds of paper are registered against the inventorydepletion of the non-standard paper, the user has to make choice thekind of paper to use for the automatic tone gradation correction. Withincrease of the number of the prepared kinds, the operation becomes moredifficult, and the probability of mistake increases. In addition, thememory capacity for the registration has to be large correspondinglywith the result of the increase in cost

Furthermore, the user is required to carry out twice the readingoperations each including producing tone gradation correction data onthe basis of reading of the printed patch on the standard paper,printing a patch on non-standard paper using the produced tone gradationcorrection data, reading the patch by the image reading station, andmaking correction. Therefore, the usability is not sufficient.

Moreover, the coloring material of electrophotographic image is low inthe transmission factor, and therefore, a solid image density issubstantially immune to paper density. The conventional automatic tonegradation correction utilizing the density detection method in theprinting field as it is, when the same amounts of the coloring materialare on different sheets of paper, the relative densities are different.For this reason, when the automatic tone gradation correction iseffected with different paper density, the result of the density aftercorrection is different, and therefore, the automatic tone gradationcorrection with the standard paper is required.

DISCLOSURE OF THE INVENTION

Accordingly, it is a principal object of the present invention toprovide an image forming apparatus in which image defects are suppressedto accomplish satisfactory image qualities by effecting engine tonegradation correction with high usability and with low memory costwithout using standard paper.

According to an aspect of the present invention, there is provided animage forming apparatus comprising an image reading station for readinga pattern image formed on a recording material by said image formingstation; and a tone gradation corrector for calculating a halftone dotarea property on the basis of a density of the pattern image read bysaid image reading station and executing tone gradation correction ofsaid image forming station on the basis of the halftone dot areaproperty.

According to another aspect of the present invention, there is providedan image forming apparatus comprising an image forming station forforming a pattern image on a recording material; an image readingstation for reading a pattern image formed on a recording material bysaid image forming station; and a tone gradation corrector for executingtone gradation correction in said image forming station on the basis ofa pattern image read by said image reading station, wherein said tonegradation corrector effects tone gradation correction processing to adensity property when a tone gradation correction for a copy image iseffected, and said tone gradation corrector effects tone gradationcorrection processing to a halftone dot area property when the tonegradation correction for a printer image is effected.

EMBODIMENT 1

FIG. 1 is a schematic illustration of an image forming system includingan image forming apparatus. In FIG. 1, the image forming system includesa host computer 1001 and an image forming apparatus 1030. The hostcomputer 1001 and the image forming apparatus 1030 are connected witheach other. FIG. 2 is a substantial illustration of an image formingapparatus 1030 according to an embodiment of the present invention.

Description of Image Forming Apparatus:

Referring to FIG. 1, a printer controller 1031 controls the entireoperation of the printer in the image forming apparatus 1030 of thisembodiment. The host I/F portion 1048 in the printer controller 1031controls the input/output relative to the host computer 1001.

An input/output buffer 1032 sends data to and receives data from acontrol code and each of communicating means, the CPU1033 controls theoperations of the printer controller 1031.

The program ROM 1034 is capable of including a control program to beexecuted by the CPU1033 and/or control data. The program ROM 1034includes as program modules an image information generation portion1041, a patch generation portion 1044, an engine tone gradationcorrection table preparing portion 1045 and an engine tone gradationcorrection executing portion 1042. These program modules are usable tocontrol conversion to brightness information and/or density informationhalftone dot area percentage, and generation of a patch image, in theexecution of the tone gradation correction which will be describedbelow, in cooperation with the CPU1033

An image information generation portion 1041 is capable of generatingvarious image objects on the basis of data setting received from thehost computer 1001. The patch generation portion 1044 is capable ofgenerating a patch image used when measuring the halftone dot areapercentage during the execution of the engine tone gradation correction.The engine tone gradation correction table preparing portion 1045 iscapable of producing the engine tone gradation correction table on thebasis of the measurement result of the halftone dot area percentage. Inaddition, engine tone gradation correction executing portion 1042 iscapable of effecting the engine tone gradation correction on the basisof the result of the measurement of the halftone dot area percentage ofthe patch image.

A RAM 1035 is capable of using for interpretation of the data,calculation necessary for printing or capable of using as work memoryfor processing the print data. In the RAM 1035, an engine tone gradationcorrection table storing portion 1050 for storing correction table canbe stored.

A bit map image expanding and transferring portion 1040 in the printercontroller 1031 expands the image object into the bit map image and iscapable of transferring the expanded bit map image to the engine portion1036 of the printing apparatus.

The engine portion 1036 of the printing apparatus includes an enginecontroller 1049 and is capable of actually effecting printing of the bitmap image expanded by the bit map image expanding and transferringportion 1040, on a sheet of paper. The engine controller 1049 is capableof controlling printing process (sheet feeding process, for example) bythe mechanisms.

The engine portion 1036 of the printing apparatus and the printercontroller 1031 are connected through the engine I/F portion 1046.

The operation of the printing apparatus can be controlled throughoperation panel 1037, and the printer controller 1031 and the operationpanel 1037 are connected with each other.

In addition, external memory portion 1038 can be used to store printdata and various information or the like of the printing apparatus. Theprinter controller 1031 and external memory portion 1038 are connectedby the memory I/F portion 1039. Each unit in the printer controller 1031is connected to the system bus 1043.

FIG. 2 is a schematic illustration of the four full color laser beamprinter of an electrophotographic type which is an image formingapparatus according to an embodiment of the present invention.

The laser beam printer (image forming apparatus of this embodimentcomprises four image forming stations for forming magenta, cyan, yellowand black images. Each of the image forming stations comprises anelectrophotographic photosensitive member (photosensitive drum) 1 a, 1b, 1 c, 1 d in the form of a drum which is an image however havingmember supported rotatably in the clockwise direction in the Figure. Thephotosensitive drum 1 a, 1 b, 1 c, 1 d is rotated counterclockwisely inthe Figure at a predetermined process speed (peripheral speed). Aroundthe photosensitive drum 1 a, 1 b, 1 c, 1 d, there are provided a primarycharger (charging means) 2 a, 2 b, 2 c, 2 d, a developing device(developing means) 4 a, 4 b, 4 c, 4 d arranged substantially in thisorder along the rotational direction. In addition, around thephotosensitive drum 1 a, 1 b, 1 c, 1 d, there are provided a transfercharger (transferring means) 5 a, 5 b, 5 c, 5 d and a cleaning device(cleaning means) 6 a, 6 b, 6 c, 6 d and so on.

Above the photosensitive drum 1 a, 1 b, 1 c, 1 d, there is provided anexposure device (exposure means) 3 a, 3 b, 3 c, 3 d for image exposureof the photosensitive drum 1 a, 1 b, 1 c, 1 d.

In each of the image forming stations, the photosensitive drum 1 a, 1 b,1 c, 1 d is uniformly charged by the primary charger 2 a, 2 b, 2 c, 2 d,and is exposed by the exposure device 3 a, 3 b, 3 c, 3 d, so that anelectrostatic latent image is formed on the photosensitive drum 1 a, 1b, 1 c, 1 d. The electrostatic latent image is visualized by thedeveloping device 4 a, 4 b, 4 c, 4 d into a toner image.

In the following, when the members or devices are referred to as a wholeor when it is not necessary to specify the colors, they will be referredto simply as photosensitive drum 1, primary charger 2, exposure device3, developing device 4, transfer charger 5 and cleaning device 6, forexample.

As shown in FIG. 2, a toner amount sensor (toner amount detecting means)30 (30 a, 30 b, 30 c, 30 d) is provided to oppose to the photosensitivedrum 1 (1 a, 1 b, 1 c, 1 d) in each of the image forming stations. Thetoner amount sensor 30 is to detect the toner amount of a toner imageformed on the photosensitive drum 1, and is used to determine a maximumtoner amount condition (grid bias of the primary charger, developingbias voltage, laser power).

Below between the developing device 4 a, 4 b, 4 c, 4 d and the cleaningdevice 6 a, 6 b, 6 c, 6 d of the photosensitive drum 1 a, 1 b, 1 c, 1 d,there is provided a transfer belt 17 which is a recording materialfeeding means in contact with the photosensitive drum 1 a, 1 b, 1 c, 1d. The transfer belt 17 rotates in the direction of an arrow R17,carrying a recording material P such as paper, transparent film or thelike to feed the recording material P to the photosensitive drum 1 a, 1b, 1 c, 1 d sequentially. The toner images formed on the photosensitivedrums 1 a, 1 b, 1 c, 1 d in the image forming stations are sequentiallytransferred onto the recording material P on the transfer belt 17 by thetransfer chargers 5 a, 5 b, 5 c, 5 d.

The recording material P is ordinarily paper, and therefore, therecording material P may be called simply paper in the followingdescription and claims, but it is not limited to paper.

The image forming apparatus further comprises a plurality of sheetfeeder, more particularly sheet feeding cassettes 12, 13, 14, a manualinsertion sheet feeding tray 11 which is drawable in the direction of anarrow R11 in FIG. 2, and a large capacity paper deck 15. The recordingmaterial P is supplied to the feeding belt 17 through one of a sheetfeeding roller, a feeding roller, a registration roller 16 of the sheetfeeders.

Onto the recording material P, the color images formed on thephotosensitive drums 1 a, 1 b are sequentially transferred, whilepassing through the image forming stations on the transfer belt 17.After the transfer step, the recording material P is separated from thetransfer belt 17 by a separation charger 18 and is fed to the fixingdevice 20 by feeding belt 19 which is a recording material guidingmeans.

The fixing device 20 comprises a fixing roller 21 supported rotatably, apressing roller 22 which rotates in press-contact with the fixing roller21, a parting material applicator 23 which is a parting material applymeans, and a roller cleaning device. Inside the fixing roller 21 andpressing roller 22, there are provided heaters (unshown) such as halogenlamps, respectively. To the fixing roller 21 and the pressing roller 22,thermisters (unshown) are contacted, respectively, and the voltagesapplied to the respective heaters through the temperature controlapparatus 26 are controlled to effect surface temperature control forthe fixing roller 21 and pressing roller 22. The pressing value of thepressing roller 22 and the surface temperatures of the fixing roller 21are variable by a fixing controlling mechanism 25.

A driving motor (unshown) for driving the fixing roller 21 and thepressing roller 22 is connected with a speed control device 27 forcontrolling rotational speeds of the fixing roller 21 and the pressingroller 22 thereby controlling the feeding speed of the recordingmaterial P. By this, the unfixed toner image on the surface of therecording material P is fused and fixed so that a full-color image isformed on the recording material P. The recording material P on whichthe full-color image is fixed is separated from the pressing roller 22by a separation claw (unshown) and is discharged onto a sheet dischargetray 24.

In the upper portion of the image forming apparatus shown in FIG. 2,there are provided an original reading station (image reading station)28 and an operation display screen 29. The original reading station 28optically scans the original supported on an original carriage (unshown)to read it and provides image signal of each color. In addition, theoperation display screen 29 is used for inputting commands by theoperator (user, service person) and for notifying the state of theapparatus to the operator. Using the reading apparatus, the automatictone gradation correcting pattern outputted from the image formingapparatus is detected, and the LUT for the engine tone gradationcorrector is changed, as will be described hereinafter.

Toner Amount Detecting Means:

FIG. 3 illustrates an example of a toner amount detection sensor whichis a toner amount detecting means. The toner amount detection sensor 30comprises a light receiving portion 401 including a light emittingportion 400 having a LED (light emitting diode), and PDs(photo-detectors). The light receiving portion 401 includes two PDs anddetects diffused reflection light.

The light Io projected to the photosensitive drum 1 from the lightemitting portion 400 is reflected by the surface of the photosensitivedrum 1. The reflected light Ir is received by the light receivingportion 401, and outputs receipt light quantity information. Thephotosensitive drum 1 used in this embodiment is a drum having a smoothsurface. When the drum is covered with the toner, the surface isnon-smooth. That is, the received light quantity increases with increaseof the toner amount. Using the change of the sensor voltage output, thetoner amount can be detected, and therefore, it can be used for themaximum toner amount control which will be described hereinafter.

The reflected light measured by the light receiving portion 401 ismonitored also by a LED light quantity controller 403. The LED lightquantity controller 403 sends the light quantity of the reflected lightIo to the main control CPU311. Prior to the maximum toner amountcontrol, the main control CPU311 adjusts the light quantity on the basisof the receipt light quantity (measured value) of the reflected light Irso that the Ir is a predetermined value.

When the image outputting operation is not carried out, a shutter drivecontrol portion 407 is operated to drive the shutter portion 408 toavoid sensor port contamination by the toner scattering.

Maximum Toner Amount Control: (Potential Control)

A potential control which is the fundamental of the maximum densitycondition will be described.

The maximum density condition is to determine charging, latent image anddeveloping conditions.

Prior to the toner amount sensor which will be described hereinafterdetects the patch, target charged potential (VdT), grid bias voltage anddeveloping bias voltage (Vdc) are determined by the potential control.By the potential control process, the charged potential or the like canbe determined corresponding to the ambient condition (includingtemperature and/or humidity conditions) in which the image formingapparatus 1030 is placed.

In this embodiment, the engine controller 1049 effects the potentialcontrol which is so-called two-point control. FIG. 4 illustrates aconcept of the two-point control.

In FIG. 4, Vd1 is a charged potential in a first charging condition(grid bias is 400V), and V11 is a light portion potential provided bythe laser power of the standard level (middle value in the laser powervariable range). In addition, Vd2 is a charged potential in a secondcharging condition (grid bias is 800V), and V12 is a light portionpotential provided by the standard laser power at this time. a In thiscase, the contrast potentials (Cont1, Cont2) with the grid bias voltagesof 400V and 800V can be calculated by following (1) and (2).

(Cont1)=(Vd1−V11)  (1)

(Cont2)=(Vd2−V12)  (2)

The increase amount of the contrast potential (ContΔ) per chargedpotential 1V can be calculated by the following (3) on the basis ofresults of (1) and (2).

(ContA)=((Cont2−Cont1)/(Vd2−Vd1))  (3).

On the other hand, the image forming apparatus 1030 is provided with anunshown ambient condition sensor, which measures the ambient conditionssuch as temperature and humidity in the image forming apparatus 1030.The engine controller 1049 determines the ambient condition (absolutewater content, for example) on the basis of the result of measurement ofthe ambient condition sensor. Then, the target contrast potential(ContT) corresponding to the ambient condition is obtained from theambient condition table provided beforehand.

The relation between the target contrast potential (ContT) and theincrease amount (ContA) of the contrast potential can be calculated bythe following (4):

ContT=Cont1+X*ContA  (4)

The parameter X satisfying the relation of equation (4), and then thetarget charged potential (VdT) (target potential) can be calculated bythe following equation (5).

VdT=Vd1+X  (5).

The change amount (VdA) of the charged potential per iV of grid biasvoltage can be calculated by the following (6)

(VdΔ)=(Vd2−Vd1)/(800−400)  (6).

The grid bias voltage (Y) for the target potential (VdT) can becalculated from the following equation (7):

Target VdT=400+Y*VdΔ  (7)

In equation (7), Vd can be calculated by equation (6), and VdT can becalculated from equation (5) By substituting a potential obtained byequations (5) and (6), the grid bias voltage (Y) satisfying equation (7)can be determined finally.

By the foregoing process, the target potential (VdT), the grid biasvoltage (Y) corresponding to the ambient condition can be determined.The developing bias voltage (Vdc) has a predetermined potential, and canbe calculated by deducting the predetermined potential from the targetpotential (VdT).

The subsequent image forming operations are carried out with thedetermined developing bias voltage. The potential of the surface of thedrum is negative, but the negative sign is omitted for easyunderstanding of the calculation process.

Thus, the grid bias voltage and the developing bias voltage (Vdc) to beused during image formation are determined.

(Maximum Toner Amount Control Using Toner Amount Sensor)

The maximum toner amount control using the toner amount sensor (toneramount detecting means) will be described.

The condition determined by the potential control matches the targetcontrast potential corresponding to the ambient condition obtained fromthe ambient condition table registered beforehand. This is the targetcontrast obtained with a standard machine, and therefore, the maximumdensity is not the predetermined level in many cases due to thedifference in the machines or after long term use. In view of this, inthis embodiment, there is provided a toner amount sensor 30 to detectthe toner amount on the photosensitive member. For the purpose ofadjusting the maximum density condition in this embodiment, the laserpower (LPW) is changed.

A pattern shown in (a) of FIG. 5 wherein the maximum toner amount isvaried is formed on the photosensitive member. For example, the patchesare formed with light quantities of the standard value, 20% Down level,10% Down level, 10% Up level, 20% Up level. The patch potentials in thiscase are as shown in (b) of FIG. 5, wherein v11 matches LPW1 (20% UP),and there are V12-V15 patches.

The size of the patches is 40 mm×40 mm square in view of a detectionrange of the potential sensor. The patch is formed with PWM (pulse widthmodulation: light emission time), and for the area between the patches,PWM is zero, and for the patch portions, PWM is adjusted forpredetermined light quantities. In this embodiment, the light is emittedwith longest duration per unit pixel (single pixel in 600 dpi).

The patch potential is measured for each LPW, and the toner amount isdetected by the toner amount sensor 30. FIG. 6 shows Vcont (Vdc−V1) ofeach patch and a patch detected value of the toner amount sensor. Inthis embodiment, the target is 0.55 mg/cm², and from the detectionresult of FIG. 6, the relation of the toner deposition amount isobtained, and then they are plotted with linear interpolation. Then, theVcont corresponding to 0.55 mg/cm² is obtained. In the case of theengine in the state shown in FIG. 6, the proper Vcont is 230V, and byincreasing the LPW by 9% (+9% Up), the toner deposition amount can bemade to 0.55 mg/cm².

The calculating method for the maximum toner amount control issummarized as follows:

-   -   preliminary preparation: Vd is set to Vdc by the potential        control:

(1) to form a latent image of a patch with the predetermined LPW (5point) (charging, laser):

(2) to detect the patch potential by the potential sensor:

(3) to develop the patch latent image:

(4) to detect the patch density by the toner amount sensor:

(5) to obtain LPW, patch potential (V1), toner amount (To):

(LPW, V1, To)=

(102 level, 150V, 0.42 mg/cm²)

(115 level, 130V, 0.45 mg/cm²)

(128 level, 100V, 0.49 mg/cm²)

(141 level, 80V, 0.56 mg/cm²)

(154 level, 55V, 0.59 mg/cm²):

(6) to convert V1 to Vcont (Vdc−V1)

(LPW, Vcont, To)=

(102 level, 165V, 0.42 mg/cm²)

(115 level, 185V, 0.45 mg/cm²)

(128 level, 215V, 0.49 mg/cm²)

(141 level, 235V, 0.56 mg/cm²)

(154 level, 260V, 0.59 mg/cm²)

(7) to calculate the difference (To−0.55) from the target tonerdeposition amount.

(LPW, Vcont, ΔTo)=

(102 level, 165V, −0.13 mg/cm²)

(115 level, 185V, −0.10 mg/cm²)

(128 level, 215V, −0.06 mg/cm²)

(141 level, 235V, 0.1 mg/cm²)

(154 level, 260V, 0.4 mg/cm²):

(8) to extract the condition for minimum positive difference, and thecondition for minimum negative difference:

Positive side (LPW (+), Vcont (+),ΔTo (+))=(141 level, 235V, 0.01mg/cm²)

Negative side (LPW (−), Vcont (−),ΔTo (−))=

(128 level, 215V, −0.06 mg/cm²)

(9) to calculate LPW (LPW (T)) at which To is 0.00

LPW (+)−((ΔTo (+)−0)/((ΔTo (+)−ΔTo (−)/(LPW (+)−LPW (−))))=139.14286

By rounding,

=139

(10) to register the potential control target:

Vcont (+)−((Vcont (+)−Vcont (−))/(LPW (+)−LPW (−))*((LPW (+)−LPW(T))=231.9231

By rounding,

=232

The registration of the potential control target in (10) is the changeof the potential control target determined by the ambient conditiontable. The maximum toner amount control using the toner amount sensorrequires printing of the patch, and from the standpoint of the tonerconsumption amount, it is not possible to increase the frequency of suchprintings. The potential control does not require development of thetoner for the detection of the patch potential, and therefore, a shorttime variation can be compensated for by the potential control.Frequency will be described.

High frequency: potential control.

Intermediate frequency: maximum toner amount control.

Low frequency (user starting): automatic tone gradation correction whichwill be described hereinafter.

<Automatic Tone Gradation Correction>

In a conventional automatic tone gradation correction, a tone gradationpatch image printed on paper is read by a reader portion (image readingstation) 28 to detect a brightness value, and the read is converted todensity information using a brightness density conversion table preparedbeforehand, and then, a LUT is adjusted so as to provide a predetermineddensity curve. As described hereinbefore, even if the same amounts ofthe toner are deposited, the densities are different depending on thekinds of paper. When the control is effected so as to match thepredetermined density curve, the inconveniences occurs in the tonegradient of the shadow portion (Background Art).

This phenomenon-is caused by following reasons generally:

(a) the color of paper (spectral characteristics) is different.

(b) the degree of melting is different depending on the paper thickness.

(c) the paper surface of property (unsmoothness knurled pits andprojections) is difference.

The density is obtained by calculating a percentage of the light (Ii) at0 degree when the light (Io) is incident at 45 degrees, and the densityincreases with decrease of the diffused reflection component.

Density=−log(Ii/Io).

As regards (a), as shown in FIG. 7, even if the same amount of the sametoner is deposited with the same pattern, the reflectance is affected bythe paper whiteness (spectral reflectance (FIG. 7C)). In the case ofFIG. 7, the non-standard paper exhibits a high density. Therefore, thedensity detected by the reader portion 28 is influenced.

As regards (b), FIG. 8 schematically explains the phenomenon. An imageforming apparatus accepts a predetermined range of basis weight, forexample, 64-128 g/m². Depending on the difference in the thickness, evenif the same amount of the same toner deposited on the paper, the heatquantity and the pressure applied to the toner in the fixing device aredifferent. Therefore, with increase of the thickness of paper, theshapes of the toner particles become closer to spherical with the resultof increase of the diffused reflection component. In the case ofstandard paper of FIG. 8, the specular reflection component increases,and the component decreases. Thus, the diffused reflection amountchanges, and therefore, the density detected by the reader portion 28 isinfluenced.

As regards (c), as shown in FIG. 9, the toner is deposited so as tofollow the shape of the unsmoothness of the paper. Therefore, when thepaper is smooth, the surface of the toner particles is also smooth, andif the smoothness of the paper is poor, the unsmoothness is reflected.Even if the deposited amount of the toner is the same, the densitydetected by reader portion 28 is different since the amount diffusedreflection is different depending on the unsmoothness of the papersurface.

Furthermore, the offset printing and the electrophotographic typeprinting are significantly different in the transmission density. Thetransmission density is lower in the offset printing than in theelectrophotographic type printing. In other words, the background(paper) appears more.

The inventors have investigated the transmission density of black usingimagePressC1 available from Canon-Kabushiki Kaisha, Japanese and anoffset printing machine with CLC sheet of 81.4 g/m². The usedtransmission densitometer is X-Rite361t. The conditions other than thesheet in the offset printing machine are JapanColor reference. Moreparticularly, the adjustments were made to meet the JapanColor withJapanPapar, and solid patches were printed on the CLC sheet.

The following Table shows transmission densities of the ink and toner,which are obtained by deducting the absolute value of the transmissiondensity of the paper from the absolute value of the transmission densityof the solid patch.

TABLE 1 Machines Offset-printer Conditions imagePressC1 Other than paperare JapannColor Transmission 0.78 1.30 Density

As will be understood, the transmission density of the toner in theelectrophotographic type exhibits high transmission density, that is,the transmission factor is low. This is because of the colorantmaterial, dispersion property of the colorant, thickness of the bondinglayer. In the case of the offset printing, the thickness of the bondingmaterial layer is 1 μm-2 μm, whereas in the case of the toner, it is 5μm-10 μm. The difference is reflected as the difference in thetransmission density.

As described, the electrophotographic type printing and the offsetprinting are different in the influence around the solid image density,but the automatic tone gradation correction has been effected using therelative density. The relative density is obtained by deducting thepaper density uniformly over the entire density area, and therefore, thededuction is too much around the high density.

In this embodiment, the density information is converted to a halftonedot area ratio (halftone dot area percentage, or simply halftone dotpercentage) so that the engine tone gradation correction can be carriedout even when the density detection result is different depending on thekind of paper.

By using the halftone dot percentage, the influence of the paper densitycan be large in the high light portion, and can be small in the shadowportion.

One predetermined target of the halftone dot percentage is provided, anda LUT is prepared so as to meet the halftone dot percentage thereof.With this structure, that is, by changing the degree of influence of thereflectance of the paper depending on the tone gradation and bycorrecting to provide the predetermined tone gradation property, smoothtone gradation property can be provided substantially free fromnon-continuous tone gradation property of the shadow portion withoutmemory cost increase, thus providing a high image quality image formingapparatus.

Referring to a flow chart of FIG. 10, the automatic tone gradationcorrection of this embodiment will be described.

The automatic tone gradation correction is started by the user orservice person pushing a tone gradation correction key on an operationscreen shown in FIG. 11.

When the tone gradation correction key is pushed a, the image formingapparatus completes the potential control and the maximum toner amountcontrol.

The patch generation portion of the program ROM 1034 prints, on thepaper, 64 tone gradation test pattern including CMYK colors shown inFIG. 12, and the reader portion 28 reads the brightness signal of thepatch. The densities of the tone gradation test pattern is such that theupper right end portion has a highest density, the lower left endportion has a lowest density, the density gradually decreases from theright to the left, and decreases from the top to the bottom. Four linesare one set for one color, and four sets are prepared for cyan, magenta,yellow and black colors.

The read signals form a curve as shown in FIG. 13. In the graph of thisFigure, the abscissa represents input signals, and ordinate representsthe brightness values read by the reader. Brightness value 255 of thereader portion 28 corresponds to density of 1.60. (the density iscalculated by Xrite500 series with StatusT, white packing, Visualfilter, absolute value base) In the description, the black (K) is takenas a representative.

FIG. 14 are plots of the brightness values of the reader portion 28 vsthe regulated density values of patch images formed on the standardpaper, wherein the density is regulated on the basis of 255corresponding to 1.60. Using the brightness density conversion table,the detected brightness value is converted to density value.

FIG. 15 shows a tone gradation property of a printer using thebrightness density conversion table of FIG. 14 relative to the inputsignal wherein the abscissa represents input signal value, and theordinate represents the density value. In a conventional image formingapparatus, the density information is converted to a relative density bydeducting the paper density therefrom, and the LUT is prepared so as tomeet the desired relative density target.

Referring to FIG. 16, the density value of FIG. 15 is converted to ahalftone dot area percentage using the Murray-Daviess formula which willbe described hereinafter, and is plotted relative to the input signal.By such a conversion to the halftone dot area percentage informationusing the Murray-Daviess formula, the maximum density portion is 100% inthe halftone dot area percentage even if the maximum read densitychanges depending on the kind of paper. Furthermore, the density of thepaper is 0%, the color of the background is not influential, and thedensity difference due to the paper kind difference can be compensatedfor over the entirety of the tone gradation.

The graph is based on halftone dot area 100% corresponding to 255.

Murray-Daviess Formula:

Halftone dot area percentage=(1−10^((−Dt)))/(1−10^((−Ds)))*100

Ds=(maximum density−paper density)

Dt=(halftone dot density−paper density)

The halftone dot density is a measurement density of the tone gradationpattern, and the maximum density is the maximum density of the tonegradation pattern.

FIG. 17 shows the printer tone gradation property prepared on the basisof the halftone dot area percentage of FIG. 16, a target curve, and aLUT for providing the target. In this embodiment, the target curve issuch a tone gradation curve that when the input signal (input halftonedot area percentage) is 50%, the output signal, that is, the outputhalftone dot area percentage is 70%, namely, the dot gain property is20%.

The LUT is stored in an engine tone gradation correction table storingportion 1050, and is used by the engine tone gradation correctionexecuting portion 1042 during normal image formation.

FIG. 18 shows a density tone gradation property using the LUT (halftonedot area percentage target) of FIG. 17, and a density tone gradationproperty using the LUT prepared from the conventional density targetprepared using the same non-standard paper.

With the conventional method, the tone gradation non-continuous propertyis unavoidable at the portions indicated by (a) and (b) since theadjustment is to meet the density value.

On the other hand, by preparing the LUT so as to meet the predeterminedhalftone dot area percentage target on the basis of the halftone dotarea percentage, the difference in the sheet can be accommodated overthe entirety of tone gradation range. That is, the degree of influenceof the reflectance of the paper is changed depending on the tonegradation and is corrected to the predetermined tone gradation property,by which the automatic tone gradation correction control immune to theinfluence of the paper. The predetermined tone gradation property istone gradations having a dot gain property within the predeterminedrange. The dot gain property is a difference between the input halftonedot area percentage and the output halftone dot area percentage.

(Verification of Embodiment 1)

The automatic tone gradation correction using the Murray-Daviess formulaof Embodiment 1 will be described. In the verification, it will beconfirmed that the object of the present invention is accomplished whilechanging the tone gradient and the influence degrees of the backgrounddepending on the tone gradation.

Paper 1: Canon-office planner 68 m² (plain paper)

Paper 2: color paper 68 g/m² pink (color paper) for Canon-PPC.

Pattern printing kind of device: ImagePress C1

Color: black.

Processing method: the automatic tone gradation correction of Embodiment1.

FIG. 19 shows a relation between the input signal (input halftone dotarea percentage) of the printing under the above-described conditionsand the density. The solid line indicates paper 1, and the broken lineindicates paper 2. They are under the same conditions, and thethicknesses thereof (basis weight) are the same, and therefore, thedensity of the solid image is substantially the same. However, the highlight portion is influenced by the color of the paper, and in the caseof the color paper, the density is approx. 0.2, and in the case of plainpaper, the density is approx. 0.07, that is, there is a difference of0.13.

FIG. 20 shows the halftone dot area percentage converted from the dataof FIG. 19 using the Murray-Daviess formula. As a result, there is nodifference depending on the paper difference. That is the paper densitydifference 0.13 has been removed. Above-described in Embodiment 1, onlyone halftone dot percentage target is used, and the same LUT isproduced. FIG. 21 shows the result which is expressed by a dot gainproperty (dot gain %) which is a difference between the input halftonedot area percentage and the output halftone dot area percentage. Thesame LUT are successfully usable for the different sheets of paper.

In the conventional example, when the color paper is used at the time ofautomatic tone gradation correction using the relative density target,the solid image density is deemed as being lower. That is, the densitydifference 0.13 between the white paper and the color paper is appliedto the solid image density. As a result, the density is lower thanexpected, and the LUT for the non-standard paper B shown in FIG. 29 isproduced. Even if a terminal end correction is carried out, thediscontinuities at the inflection points are unavoidable.

With the automatic tone gradation correcting method, the natural tonegradient without the discontinuity can be accomplished even when thereis an extreme difference, as in the case of color paper.

In another aspect, the influence of the paper does not appear in thesolid density portion. On the other hand, in the high light portion, thedifference of the kind of the paper is accommodated by the calculatingprocess. In other words, the degree of influence of the density of thepaper (influence of reflectance of paper) is modified depending on thetone gradation (halftone dot percentage). The influence degrees of thepaper is high in the high light portion, and is low in the shadowportion, by which an automatic tone gradation correcting method immuneto the different of the paper is accomplished.

EMBODIMENT 2

In Embodiment 2, the Murray-Daviess formula is modified to provide amore versatile automatic tone gradation correcting method. The processwhich is the same as with Embodiment 1 will be omitted.

Paper 1: Canon-office planner 68 m² (plain paper)

Paper 2: color paper (thick type) 125 g/m² blue (color paper) usingCanon-PPC.

Pattern printing kind of device: ImagePress C1

Color: black.

Processing method: the automatic tone gradation correction of Embodiment1.

The paper 1 is similar to the paper used in the verification inEmbodiment 1, and the paper 2 is different in color and thickness (basisweight). Since the phenomenon-shown in FIG. 8 with respect to Embodiment1 occurs, the solid image density in the case of paper 2 is low (FIG.22). The density difference of paper is approx. 0.1.

Under these conditions, the automatic tone gradation correction ofEmbodiment 1 is carried out, and density of the output patch image isanalyzed, the result of which is shown in FIG. 23.

At a glance, the matching is all right, but the deviation of halftonedot percentage occurs in the half-tone range. In consideration of theresult, the LUT is prepared so as to meet the halftone dot percentagetarget, and the dot gain property is investigated, and then the resultof the investigation is shown in FIG. 24 wherein the accuracy is poorerthan in the case of the color paper used in the verification inEmbodiment 1.

As compared with the conventional example, the tone gradient isimproved, but the result of the engine tone gradation correction isdifferent depending on the kind of paper. For this reason, the automatictone gradation correction is carried out with different sheets of paper(paper 1 and paper 2), when printing is effected on another paper, adensity difference occurs in the half-tone range. The facts show thatwhen the density of the paper and the solid image density are bothdifferent from white, the accuracy of the Murray-Daviess formula islimited.

The Murray-Daviess formula is as follows in which Ds and Dt are the sameas with the conventional relative densities. Therefore, the densitydifference in the shadow portion could not be compensated for.

Murray-Daviess Formula:

Halftone dot area percentage=(1−10^((−Dt)))/(1−10^((−Ds)))*100

Ds=(maximum density−paper density)

Dt=(halftone dot density−paper density)

In this embodiment, the calculation is effected in two stages toaccommodate a density difference in the shadow portion.

First Processing:

First halftone dot area percentage=(1−10^((−Dt1)))/(1−10^((−Ds1)))*100

Ds1=(maximum density−paper density)

Dt1=(halftone dot density−paper density).

Second Processing.

Halftone dot area percentage=(1−10^((−Dt)))/(1−10^((−Ds)))*100

Ds2=(maximum density−paper density*(100−first halftone dot areapercentage))

Dt2=(halftone dot density−paper density*(100−first halftone dot areapercentage))

The first calculation is the same as with Embodiment 1 using theMurray-Daviess formula to obtain a first halftone dot area percentagewhich is modified by weighting relative to the paper density by a secondcalculation.

FIG. 25 shows a result of the second calculation. It has been confirmedthat the curves are the same in the halftone dot area percentage. If thehalftone dot area percentage target is prepared by the two steps, thesame tone gradation correction table can be prepared irrespective of thedensity of the paper and difference in the solid image density, and thedot gain curves exhibit the same property.

In this manner, the influence degrees of the paper density are madedifferent between the high light portion and the shadow portion, and thehalftone dot area percentage is calculated, by which highly versatileautomatic tone gradation correction is accomplished.

EMBODIMENT 3

In Embodiment 1, an accurate automatic tone gradation

P. density P. density P. density P. density <0.1 <0.1 ≧0.1 ≧0.1correcting method is provided in the case that the basis weight is thesame and a density difference exists. In Embodiment 2, the automatictone gradation correcting method is versatile even when the solid imagedensity is also different due to difference of the basis weight, inaddition to Embodiment 1.

From the standpoint of calculation process or memory capacity, ascompared with, the conventional calculation of the density, Embodiment 2includes the first halftone dot percentage calculation and the secondcalculation (one conversion calculation in Embodiment 1), and therefore,Embodiment 2 results in increase in the processing time and memoryamount for temporary storage.

In Embodiment 3, the content of the calculation is changed to avoidincrease of the process time and/or the amount of the memory.

TABLE 2 Basis Wt Basis Wt Basis Wt Basis Wt <90 g/m² ≧90 g/m² <90 g/m²≧90 g/m² Conventional 0 Density *1  calculation Embodiment 1 0 0 1^(st)calculation *2  *3  Embodiment 2 0 2^(nd) calculation *4  In *1, thecondition is substantially for the reference paper, and therefore, theinconvenience due to the difference of the paper does not arise, and thecalculation is performed with the same density as with the prior art. Asregards *2, since the amount of the generation of the density differencedepending on the difference in the basis weight is smaller than thatdepending on the difference of the media density although not describedin Embodiment 1 or 2, the first calculation of Embodiment 1 isconsidered as being enough. The calculation of *3 is the same as that ofEmbodiment 1. The calculation of *4 is the same as that of Embodiment 2.

For actual switching, the shaded portion of FIG. 26 is added to theprogram of the engine tone gradation correction table preparing portionof FIG. 1.

The CPU1033 which has executed the maximum toner amount control obtainsthe property of the paper to print the 64 patch images for the automatictone gradation correction on the paper, and stores it in the RAM 1050temporarily. Similarly Embodiment 1, the reading of the 64 patch imagesby the reader portion and the brightness to density conversion arecarried out.

Among 64 pieces of patch information which are converted densityinformation, the density of the paper white portion i.e. 0 signal valueis extracted, and the discrimination is made as to whether or not it isless than 0.1. If it is less than 0.1, the basis weight is checked, andif it is less than 90 g/m², the LUT is prepared in this state. If it isnot less than 90 g/m², the conversion is made to the first halftone dotpercentage by the calculation of the Embodiment 1, and the LUT isprepared.

Since the paper density is not less than 0.1, the influence of the paperdensity is great, and therefore, the halftone dot percentage calculation(the first calculation of Embodiment 1 and Embodiment 2) is carried outIf the basis weight is less than 90 g/m², LUT is generated, and if it isnot less than 90 g/m², the second calculation of Embodiment 2 isexecuted, and then the LUT is generated.

In the process flow described above, the calculation method for the LUTcan be automatically selected depending on the necessity (the basisweight of the paper and the density of the paper)

In this embodiment, the calculation method is switched depending on theresult of the automatic detection of the paper density and the paperbasis weight, but the switching may be made in response to userinstructions.

EMBODIMENT 4

The halftone dot percentage calculation is executed in order to suppressthe density discontinuity and to provide good tone gradientreproducibility. When the original is on the screen and is printed our,the present invention is particularly effective. On the screen, theadditive color mixture by RGB is used, and on the print, the subtractivecolor mixture requiring reference light occurs (body color) Although themechanism of coloring is different, general users hardly desire strictidentity between the density on the screen and the density on the print.

However, in the case of copying, the original documents are print paperor printer outputs, that is, the colors are all body colors The outputprints are easily compared to the original in the densityreproducibility, and the faithful reproduction is desired in not a fewcases.

In this embodiment, therefore, the LUT generation method for copying isso as to adjust the tone gradation to the conventional density target,and the LUT generation for the printer is so as to adjust the tonegradation to the halftone dot area percentage target.

FIG. 27 shows this feature on the basis of Embodiment 4. Using the patchsignal after the density conversion, a conventional LUT for the copyingis generated depending on the density state. Simultaneously, thecalculation method is selected depending on the density of paper and/orthe basis weight thereof, and a predetermined halftone dot percentagecalculation (first calculation only or first plus second calculations),and the result is stored in the table storing portion as the LUT for theprinter.

In actual operation, the CPU1033 sends information indicative of whetherit is the printer or copying selected to the engine tone gradationcorrection executing portion 1042, and the image is formed using the LUTstored in the engine tone gradation correction table storing portion.

In the printer image formation of the present invention, an image objectis generated by the image information generation portion 1041 on thebasis of a print information inputted from the host computer 1001 ofFIG. 1, is converted to bit map data by the bit map image expanding andtransferring portion 1040, is subjected to the tone gradation correctionby the engine tone gradation correction executing portion 1042 in theprogram ROM 1034, and then is sent to the engine portion 1036.

On the other hand, in the copy image formation, the original is read bythe original reading station 28 of FIG. 2, is sent to the printercontroller, and is subjected to the engine tone gradation correctionexecuting portion 1042 for the tone gradation correction for copying,and then is sent to the engine portion 1036 and is outputted.

In this embodiment, the pseudo-half-tone process pattern is common tothe printer and the copying, and therefore, two LUTs are prepared, butif the pseudo-half-tone process patterns for the printer and the copyingare different from each other, it is not necessary to prepare two LUTsfor one pseudo-half-tone process pattern.

EMBODIMENT 5

In order to improve the usability, the selection may be prompted to theuser as shown in FIG. 28 and FIG. 29. In such a case, the LUT isgenerated using the conventional density in the case of the densitypreference, and Embodiment 1 or Embodiment 2 is used in the case of thetone gradation preference.

As described in the foregoing, an image forming apparatus in which thetone gradation discontinuous property of the shadow portion arising inthe automatic tone gradation correction for the non-standard paper isavoided, and the engine tone gradation correction is possible for thenon-standard paper, is provided.

In each of the embodiments, the color image forming apparatus has been adirect transfer type apparatus, but the present invention is not limitedto such an apparatus.

For example, the present invention is applicable to a color imageforming apparatus of an intermediary transfer type. In such an imageforming apparatus, the toner images are once transferred from the imagebearing members of the image forming stations onto an intermediarytransfer member such as an intermediary transfer belt, and thereafter, atoner images are transferred all together on the recording material(paper) to provide a color image.

As for the color image forming apparatus of the intermediary transfertype, the apparatus is not limited to a color image forming apparatus,either, but it may be a monochromatic image forming apparatus in whichthe automatic tone gradation correction control of said Embodiment 1, 2is implemented, by which high quality images can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating structures of an image formingsystem having an image forming apparatus according to an embodiment.

FIG. 2 is a schematic illustration of the image forming apparatusaccording to the embodiment.

FIG. 3 illustrates an example of a toner amount detection sensor.

FIG. 4 illustrates a concept a potential control.

FIG. 5 illustrates a patch image used in a maximum toner amount control.

FIG. 6 illustrates a concept of the maximum toner amount control.

FIGS. 7A, 7B and 7C illustrate difference of color of the paper.

FIG. 8 is a schematic view illustrating a relation between thedifference of the paper thickness and the reflectance.

FIG. 9 is a schematic view illustrating a relation between thedifference of the paper unsmoothness and the reflectance.

FIG. 10 is a flow chart illustrating a control manner in the imageforming apparatus according to the embodiment.

FIG. 11 illustrates an example of an operation screen of the imageforming apparatus.

FIG. 12 shows an example of an automatic tone gradation correctingpattern.

FIG. 13 is a brightness property when the automatic tone gradationcorrecting pattern is read by the reader portion.

FIG. 14 shows a relation between the brightness obtained by the readerportion.

FIG. 15 shows a density property obtained by reading the automatic tonegradation correcting pattern by the reader portion.

FIG. 16 shows a halftone dot area percentage property obtained byreading the automatic tone gradation correcting pattern by the readerportion.

FIG. 17 shows a printer tone gradation property, a LUT and a target.

FIG. 18 shows results of printer tone gradation properties in theconventional example and according to the present invention.

FIG. 19 shows a relation between an input signal in verification ofEmbodiment 1 and a density.

FIG. 20 shows a relation between the input signal in the verification ofEmbodiment 1 and a halftone dot area percentage.

FIG. 21 shows a result of the automatic tone gradation correction in theverification of Embodiment 1.

FIG. 22 shows a relation below an input signal in

Embodiment 2 and a density.

FIG. 23 shows an input signal determined by a calculating method for thehalftone dot area percentage and a halftone dot area percentage inEmbodiment 1.

FIG. 24 shows a result of the automatic tone gradation correction by thehalftone dot area percentage calculating method according to Embodiment1.

FIG. 25 shows a result of the automatic tone gradation correction inEmbodiment 2.

FIG. 26 is a flow chart illustrating Embodiment 3.

FIG. 27 is a flow chart illustrating Embodiment 4.

FIG. 28 illustrates a user interface for the tone gradation correctingprocess instructions in Embodiment 5.

FIG. 29 is an illustration of a user interface for the tone gradationcorrecting process instructions for a copier and a printer.

FIG. 30 shows inside structures of a conventional image processor.

FIG. 31 shows a color setting portion of a conventional printer driver.

FIG. 32 shows a kind of paper and a density property in a conventionalexample.

FIG. 33 shows a LUT when an automatic tone gradation correction iscarried out.

FIG. 34 shows a LUT when an automatic tone gradation correction iscarried out.

FIG. 35 shows a result of correction when the conventional automatictone gradation correction is carried out.

INDUSTRIAL APPLICABILITY

According to the present invention, there is provided an image formingapparatus with which high image quality is accomplished by suppressingimage defect by the high usability engine tone gradation correctionwithout increasing memory cost and without using a standard paper.

1. An image forming apparatus comprising: an image reading station forreading a pattern image formed on a recording material by said imageforming station; and a tone gradation corrector for calculating ahalftone dot area property on the basis of a density of the patternimage read by said image reading station and executing tone gradationcorrection of said image forming station on the basis of the halftonedot area property.
 2. An apparatus according to claim 1, wherein saidtone gradation corrector executes the tone gradation correction suchthat a dot gain property is within a predetermined range.
 3. Anapparatus according to claim 2, wherein the dot gain property is adifference between an input halftone dot area percentage and an outputhalftone dot area percentage.
 4. An apparatus according to claim 3,wherein the output halftone dot area percentage is determined by,(1−10^((−Dt)))/(1−10^((−Ds)))*100,where Ds=maximum density−paper densitydt=halftone dot density−paper density.
 5. An apparatus according toclaim 3, wherein the output halftone dot area percentage is determinedby.(1−10^((−Dt2)))/(1−10^((−Ds2)))*100where Ds2=(maximum density−paper density*(100−first halftone dot areapercentage))dt2=(halftone dot density−paper density*(100−first halftone dot areapercentage))first halftone dot area percentage=(1−10^((−Dt)))/(1−10^((−Ds))*100ds=(maximum density−paper density)dt=(halftone dot density−paper density).
 6. An apparatus according toclaim 1, wherein said image forming station forms an electrostaticlatent image on an image bearing member, visualize the electrostaticlatent image into a toner image by developing means, and transfers thetoner image on the paper.
 7. An apparatus according to claim 1, whereinsaid tone gradation corrector effects tone gradation correctionprocessing to the tone gradation property when a tone gradationcorrection for a copy image is effected, and said tone gradationcorrector effects tone gradation correction processing to the halftonedot area property when the tone gradation correction for a printer imageis effected.